Use CDN URL for public file links when S3_CDN_URL is set

getPublicUrl returns the CDN URL (files.second-brain.ru via Bunny) when
S3_CDN_URL is configured, falling back to the direct S3 endpoint. This
was a manual patch on the build server that was never committed —
restore it so new uploads also get CDN-backed (RU-accessible) URLs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/s3.ts

@@ -14,6 +14,8 @@ export const s3 = new S3Client({
 const BUCKET = process.env.S3_BUCKET!;
 
 export function getPublicUrl(key: string): string {
+  const cdn = process.env.S3_CDN_URL;
+  if (cdn) return `${cdn}/${key}`;
   return `${process.env.S3_ENDPOINT}/${BUCKET}/${key}`;
 }